The present invention relates generally to the field of lithium anodes for use in electrochemical cells. More particularly, the present invention pertains to an anode for use in an electrochemical cell comprising an anode active layer comprising lithium and a polymer anode substrate comprising a polymer film layer and a crosslinked polymer protective layer in contact with the anode active layer. The present invention also pertains to methods of forming such anodes and electrochemical cells comprising such anodes.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
There has been considerable interest in recent years in developing high energy density batteries with lithium containing anodes. Lithium metal is particularly attractive as the anode of electrochemical cells because of its extremely light weight and high energy density, compared for example to anodes, such as lithium intercalated carbon anodes, where the presence of non-electroactive materials increases the weight and volume of the anode, and thereby reduces the energy density of the cells, and to other electrochemical systems with, for example, nickel or cadmium electrodes. Lithium metal anodes, or those comprising mainly lithium metal, provide an opportunity to construct cells which are lighter in weight, and which have a higher energy density than cells such as lithium-ion, nickel metal hydride or nickel-cadmium cells. These features are highly desirable for batteries for portable electronic devices, such as cellular phones and laptop computers, where a premium is paid for low weight.
It is still more advantageous to incorporate these features into thin film designs for several reasons. For example, the large surface area inherent in a thin film cell design allows high total currents to be passed with a modest current density, thereby dramatically enhancing the power capability of the cell, as well as reducing charging time. Low current densities are especially important for lithium electrodes, since plating (recharging) lithium at high currents may result in rough, high-surface-area, forms of lithium which decrease performance and may create a safety hazard.
In order to achieve high energy density, it is important to minimize the volume and weight of non-electroactive components in a battery, for example, components such as substrates for the electroactive materials. Organic polymers possess light weight and can be readily fabricated into thin films with desirable physical properties, such as flexibility, strength, and easy processability at moderate temperatures.
However, like most organic materials, most organic polymers will react with lithium metal; including polymeric materials, such as polyesters, which have desirable physical properties as thin film battery substrates. In U.S. Pat. No. 5,360,684 to Duval et al., an insulating band separating a lithium film from a metallic cathode current collector in an electrochemical cell is described. Examples of polymers for insulating bands include polyethylene, polypropylene, or materials such as polyester, Teflon, polyamide, with a thin surface coating of polyethylene or polypropylene.
There is still a need for improved polymeric materials that are light in weight, are as thin as possible, possess flexibility, and have better resistance to reaction with lithium metal for use as substrates and other components in thin film lithium electrochemical cells and batteries.
The anode of the present invention for use in an electrochemical cell comprises: (a) an anode active layer comprising lithium; and (b) a polymer anode substrate wherein the polymer anode substrate comprises a polymer film layer and a protective crosslinked polymer layer wherein the protective crosslinked polymer layer is in contact with the anode active layer comprising lithium on the side opposite to the surface in contact with the polymer film layer.
In one embodiment, the polymer film layer is a polyester film selected from the group consisting of polyethylene terephthalate, polyethylene naphthalate, 1,4-cyclohexanedimethylene terephthalate, and polyethylene isophthalate. In one embodiment, the polymer film layer is from 1 to 25 microns in thickness. In one embodiment, the polymer film layer is from 2 to 10 microns in thickness.
In one embodiment, the protective crosslinked polymer layer is formed from the polymerization of one or more monomers selected from the group consisting of alkyl acrylates, glycol acrylates, polyglycol acrylates, and polyol polyacrylates. In one embodiment, the protective crosslinked polymer layer is formed from the polymerization of one or more monomers selected from the group consisting of 1,6-hexanediol diacrylate, tripropylene glycol diacrylate, and trimethylol propane triacrylate. In one embodiment, the protective crosslinked polymer layer is from 0.01 to 4 microns in thickness. In one embodiment, the protective crosslinked polymer layer is from 0.1 to 2 microns in thickness. In one embodiment, the protective crosslinked polymer layer is from 0.01 to 0.5 microns in thickness.
In one embodiment, the anode comprises a metal current collector layer interposed between the protective crosslinked polymer layer and the anode active layer comprising lithium. In one embodiment, the metal current collector layer comprises a metal selected from the group consisting of copper and nickel.
In one embodiment, the anode active layer comprising lithium is from 5 to 50 microns in thickness.
Another aspect of the present invention pertains to methods of preparing an anode for use in an electrochemical cell, wherein the anode, as described herein, is formed by the steps of:
(a) depositing onto a polymer film layer a polymerizable layer comprising one or more monomers;
(b) polymerizing the polymerizable layer of step (a) to form a layer comprising a protective crosslinked polymer; and
(c) depositing over the protective crosslinked polymer layer an anode active layer comprising lithium.
In one embodiment, the polymerizable layer of step (a) comprising one or more monomers is deposited by a method selected from the group consisting of flash evaporation and spin coating.
In one embodiment, the polymerization step (b) is initiated by an energy source selected from the group consisting of heat, ultraviolet light, visible light, infrared radiation, and electron beam radiation.
In one embodiment, subsequent to step (b) and prior to step (c), there is an additional step of depositing onto the protective crosslinked polymer layer, on the side opposite to the polymer film layer, a metal current collector, as described herein.
In one embodiment of the invention, after step (c) there is an additional step (d) of depositing on the anode active layer, on the side opposite from the protective crosslinked polymer layer, a single ion conducting protective layer.
A further aspect of the present invention pertains to an electrochemical cell comprising:
(a) a cathode comprising a cathode active material;
(b) an anode; and
(c) a non-aqueous electrolyte interposed between the anode and the cathode, wherein the anode comprises:
(i) an anode active layer comprising lithium; and
(ii) a polymer anode substrate, wherein the polymer anode substrate comprises a polymer film layer and a protective crosslinked polymer layer, wherein the protective crosslinked polymer layer is in contact with the anode active layer comprising lithium on the side opposite to the surface in contact with the polymer film layer.
As will be appreciated by one of skill in the art, features of one aspect or embodiment of the invention are also applicable to other aspects or embodiments of the invention.